1. Field of the Invention
The present invention relates to a method for designing non-round soft contact lenses, and more particularly to a method for designing non-round soft contact lenses with the objective of improving lens stability, rotation and centration on eye by optimizing the variation of lens-lid contact area as a function of the lens position in the centration/rotation on the wearer's eye.
2. Discussion of the Related Art
It is known that correction of certain optical defects may be accomplished by imparting non-rotationally symmetric corrective characteristics to one or more surfaces of a contact lens such as cylindrical, bifocal, multifocal, wavefront corrective characteristics or decentration of the optical zone. It is also known that certain cosmetic features such as print patterns, markings, and the like are required to be placed in a specific orientation relative to the wearer's eye. The use of contact lenses is problematic in that each contact lens of the pair must be maintained at a specific orientation while on the eye to be effective. When the contact lens is first placed on-eye, it must automatically position, or auto-position, itself and then maintain that position over time. However, once the contact lens is positioned, it tends to rotate on the eye due to the force exerted on the contact lens by the eyelids during blinking.
Maintenance of the on-eye orientation of a contact lens typically is accomplished by altering the mechanical characteristics of the contact lens. For example, prism stabilization, including decentering of the contact lens' front surface relative to the back surface, thickening of the inferior contact lens periphery, forming depressions or elevations on the contact lens' surface, and truncating the contact lens edge, are methods that have been used.
Additionally, dynamic stabilization has been used in which the contact lens is stabilized by the use of thick and thin zones, or areas in which the thickness of the contact lens' periphery is increased or reduced, as the case may be. Typically, the thick and thin zones are located in the contact lens' periphery with symmetry about the vertical and horizontal axes. For example, each of two thick zones may be positioned on either side of the optic zone and centered along the 0-180 degree axis of the contact lens. Thus, in the majority of individuals the upper eyelid, for example, will strike the uppermost end of one thick zone before it does the other. This results in tilting of the contact lens moving it away from the orientation that is sought to be maintained.
Maintenance of contact lens orientation and centration has been performed so far using mechanical features on the contact lenses but no design is currently taking advantage of the geometry of the upper and lower eyelids to adjust the shape of the contact lens outer contour in accordance with these geometries to provide improved stability in orientation, rotation and centration.
U.S. Pat. No. 7,216,978 illustrates that the upper and lower eyelids do not move strictly in a vertical direction, with an up and down stroke during blinking. The upper eyelid moves substantially vertically, with a small nasal component during blinking, and the lower eyelid moves substantially horizontally, moving nasalward during blinking. Additionally, the upper and lower eyelids are not symmetrical with respect to a plane cutting though the vertical meridian. In other words, individuals do not blink symmetrically relative to a horizontal axis drawn between the open upper and lower lid. Also, it is known that the eyes converge when the viewer gazes down to read. Accordingly, blinking in of itself may not result in the ideal translation of the contact lens.
Accordingly, it would be advantageous to develop a method for designing contact lenses that optimizes the shape of the contact lens outer contour such that the difference between the lens-eyelid contact area when the contact lens is mis-oriented and the lens-eyelid contact area when the contact lens is correctly oriented is maximized.